Distributed circular geometry power amplifier architecture

ABSTRACT

The present invention discloses a distributed power amplifier topology and device that efficiently and economically enhances the power output of an RF signal to be amplified. The power amplifier comprises a plurality of push-pull amplifiers interconnected in a novel circular geometry that preferably function as a first winding of an active transformer having signal inputs of adjacent amplification devices driven with an input signal of equal magnitude and opposite phase. The topology also discloses the use of a secondary winding that matches the geometry of primary winding and variations thereof that serve to efficiently combine the power of the individual power amplifiers. The novel architecture enables the design of low-cost, fully-integrated, high-power amplifiers in the RF, microwave, and millimeter-wave frequencies.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.60/239,470 and 60/239,474, both filed on Oct. 10, 2000 and U.S.Provisional Application No. 60/288,601, filed on May 4, 2001, and is acontinuation of U.S. application Ser. No. 09/974,578, now U.S. Pat. No.6,816,012, and U.S. application Ser. No. 10/942,348, both of which arehereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to high frequency power amplifiers andmore particularly to techniques for combining, monolithically orotherwise, individual power amplifiers to achieve power combining andimpedance transformation.

2. Description of the Related Art

The design of high frequency power amplifiers with reasonable powerlevels, efficiency and gain remains one of the major challenges in thepursuit of a single-chip integrated transceiver. Although severaladvances have been made in this direction, the design of a trulyintegrated power amplifier on a lossy substrate, such as silicon orsilicon germanium has been an elusive goal.

Multiple external components such as bonding wires and external balunshave been used as tuned elements to produce output power levels inexcess of 1 W using CMOS transistors. See e.g., K. C. Tsai and P. R.Gray, “A 19 GHz, 1-W CMOS Class-E Power Amplifier for WirelessCommunications,” IEEE Journal of Solid-State Circuits, vol. 34, no. 7,pp. 962-969, July 1999 ^([1]); and C. Yoo and Q. Huang, “A Common-GateSwitched, 0.9 W Class-E Power Amplifier with 41% PAE in 0.25 μm CMOS,”Symposium on VLSI Circuits Digest, pp. 56-57, Honolulu, June 2000^([2]). Similar performance levels have been achieved with Si-Bipolartransistors. See, e.g. W. Simbürger, et al, “A Monolithic TransformerCoupled 5-W Silicon Power Amplifier with 59% PAE at 0.9 GHz,” IEEEJournal of Solid-State Circuits, vol. 34, no. 12, pp. 1881-1892,December 1999 ^([3]); and W. Simbürger, et al., “A Monolithic 2.5V, 1 WSilicon Bipolar Power Amplifier with 55% PAE at 1.9 GHz,” IEEE MTT-SDigest, vol. 2, pp. 853-856, Boston, June, 2000. ^([4])

Moreover, alternative technologies for active devices with higherbreakdown voltages and higher substrate resistivity have been used toincrease the output power and efficiency of integrated amplifiers. Forexample, LDMOS transistors with a breakdown voltage of 20V have beenused on a semi-insulating substrate, but still this design delivers only200 mW. See, Y. Tan, et al., “A 900-MHz Fully Integrated SOI PowerAmplifier for Single-Chip Wireless Transceiver Applications,” IEEESolid-State Circ., vol. 35, no. 10, pp. 1481-1485, October 2000 ^([5]).Further, GaAs MESFET's on insulating substrates have been used tointegrate power amplifiers. J. Portilla, H. Garcia, and E. Artal, “HighPower-Added Efficiency MMIC Amplifier for 2.4 GHz WirelessCommunications,” IEEE Journal of Solid State Circuits, vol. 34, no. 1,pp. 120-123, January 1999 ^([6]). Unfortunately, these technologies aresignificantly more costly and more difficult to manufacture thanconventional silicon-based transistor technologies, such as CMOS.

A summary of these prior achievements in the design of high-frequency,low voltage power amplifiers is provided in Table 1:

TABLE 1 External Freq. P_(out) Supply PAE Wirebond Compo- Active Ref.(GHz) (W) Voltage % Inductor? nents? Device No. 1.9 1.0 2 41 YES YESCMOS [1] 0.9 1.0 1.9 41 YES YES CMOS [2] 0.9 5.0 4.5 59 NO YES SiBipolar [3] 1.9 1.4 2.5 55 NO YES Si Bipolar [4] 0.9 0.2 5 49 NO NO SOI[5] LDMOS 2.4 0.25 7 79 NO NO MESFET [6]

Two significant problems in the design of a fully-integrated high speedsolid state power amplifier using conventional silicon technologies suchas CMOS are (1) the low resistivity of the lossy substrate whichincreases the loss of on-chip inductors and transformers; and (2) thelow breakdown voltages of the transistors. These problems areexacerbated as the minimum feature sizes of the transistors (such asCMOS) are scaled down for faster operation.

More particularly, the high conductivity of lossy substrates causes longmetal lines, including conventional spiral inductors fabricated on thesame substrate, to be very lossy in terms of power. If the metal linesare made wide to reduce resistance, the capacitive coupling effectbetween the metal and substrate will drain part of the current to thesubstrate, thereby increasing power dissipation. On the other hand, ifthe metal lines are made narrow enough to effectively overcome thisproblem, the metal resistance will significantly increase, againabsorbing (dissipating) a significant portion of the power.

The low breakdown voltage of conventional transistors such as CMOS, forexample, limits the maximum allowable drain voltage swing of thetransistor. This makes it necessary to perform some form of impedancetransformation to achieve a larger output power. For example, a ±2Vdrain voltage swing delivers only 40 mW to a 50Ω load if no suchimpedance transformation is performed. While impedance transformationcan be achieved using a 1:n transformer, unfortunately, an on-chipspiral 1:n transformer on a standard CMOS substrate is very lossy andwill degrade the performance of the amplifier greatly. Alternatively, anon-chip resonant match could be used, but this technique also results insignificant power loss.

In sum, as all high frequency power amplifiers ostensibly require someinductors—essentially long metal lines—for matching purposes,connections for the supplies, and some form of power combining,conventional power amplifier circuits tend to be very power inefficientand not commercially viable above certain power and frequency levels.

Thus, it would be highly desirable to have a low cost, fullyintegratable topology for a power amplifier that can be fabricated withlow cost, silicon-based processes and that can provide significantoutput power levels in the microwave and millimeter-wave frequencyranges. It would also be desirable if such a topology could beimplemented with discrete power amplifiers as well as monolithicintegration techniques. Ideally, this architecture would also be usefulin the design of both lossy substrate IC's as well as non-lossysubstrate IC's.

SUMMARY OF THE INVENTION

The present invention, which addresses these needs, resides in adistributed, circular-geometry, power amplifier as a means for powercombining and impedance transformation to achieve a very high outputpower in a small package and to overcome the low breakdown voltage ofconventional active devices such as short-channel MOS transistors.

In particular, the present invention resides in a distributed, circulargeometry, power amplifier for amplifying an RF input signal thatcomprises a plurality of smaller push-pull amplifiers. Each amplifierincludes two gain blocks that each has an input port with positive andnegative terminals and an output port with positive and negativeterminals. The two gain blocks of each push-pull amplifier areinterconnected at the positive terminals of their respective outputports by an inductive path and share a common supply voltage to thepositive terminals of their respective output ports. The negativeterminal of the output port of each gain block of each push-pullamplifier is connected to negative terminal of the output port of a gainblock of an adjacent push-pull amplifier such that the amplifiers areconfigured in an interconnected circular geometry, with the connectednegative terminals of adjacent gain blocks being connected together toform a virtual ac ground.

In operation, the input port of each gain block is adapted to receive anac input signal of at least substantially equal magnitude and oppositephase relative to the input port of an adjacent gain block. Thepush-pull amplifiers are interconnected such that, for the fundamentalfrequency of operation, virtual ac-grounds are presented at the negativeterminals of the output ports of the gain blocks.

In a more detailed aspect of the present invention, the distributedcircular geometry power amplifier comprises at least two push-pullsamplifiers designed to amplify an RF input signal. A first push-pullamplifier includes a first gain block and a second gain block, eachblock having an input port with positive and negative terminals and anoutput port with positive and negative terminals, the blocks beinginterconnected at the positive terminals of their respective outputports by an inductive path. A second push-pull amplifier includes athird gain block adjacent the second gain block and a fourth gain block,the third and fourth gain block each having an input port with positiveand negative terminals and an output port with positive and negativeterminals, the gain blocks of the second push-pull amplifier beinginterconnected at the positive terminals of their respective outputports by an inductive path. In order to create the “circular” closedloop, the second and third gain blocks are interconnected at thenegative terminals of their respective output ports and the negativeterminal of the output port of the fourth gain block is connected to thenegative terminal of the output port of the first gain block such thatsubstantially all ac current that flows from the fourth gain block flowsinto the first gain block. The fourth and first gain blocks may, buttypically will not, be directly connected to each other. In a typicalconfiguration, at least (and preferable more than) one additionalpush-pull amplifier having a pair of interconnected gain blocks isprovided between the fourth and first gain block, such that the negativeterminal of the output port of the fourth gain block is indirectlyconnected to the negative terminal of the output port of the first gainblock via this at least one additional push-pull amplifier.

In a more detailed embodiment, the power amplifier further includes athird and fourth push-pull amplifier, thereby creating a quad-push-pullpower amplifier with eight gain blocks. In particular, the thirdpush-pull amplifier has fifth and sixth gain blocks, each having aninput port with positive and negative terminals and an output port withpositive and negative terminals, the fifth and sixth blocks beinginterconnected at the positive terminals of their respective outputports by an inductive path. Similarly, the fourth push-pull amplifierhas seventh and eighth gain blocks, each block having an input port withpositive and negative terminals and an output port with positive andnegative terminals, the seventh and eighth blocks being interconnectedat the positive terminals of their respective output ports by aninductive path. The quad-amplifier device is interconnected such thatthe negative terminal of the output port of the fourth gain block isconnected to the negative terminal of the output port of the fifth gainblock, the negative terminal of output port of the sixth gain block isconnected to the negative terminal of the output port of the seventhgain block, and the negative terminal of output port of the eighth gainblock is connected to the negative terminal of the output port of thefirst gain block.

The gain blocks that comprise the push-pull amplifiers used by thepresent invention may take various configurations, depending on thedesired gain, circuit complexity, cost and other factors. In one basicembodiment, each gain block of each push-pull amplifier comprises asingle three-terminal active device, such as a CMOS or bipolartransistor, having a cathode, an anode, and a control terminal. Inanother embodiment, each gain block of each push-pull amplifiercomprises a compound device having at least a first and a lastthree-terminal active device. The active devices of each gain block areconnected together in a cascoded fashion such that the cathode of thefirst active device serves as the negative terminal of the output portof each gain block, the anode of the last active device serves as thepositive terminal of the output port of each gain block, and the controlterminal of the first active device is the input port of the gain block.With this configuration, each push-pull amplifier, and thus the poweramplifier, can advantageously supply more gain than can a singletransistor per gain block design.

The power amplifier of the present invention enables the push-pullamplifiers to be monolithically integrated onto a single chip. Moreover,the inductive path of each push-pull amplifier may simply be a metalslab and more particularly a substantially straight metal slab.

In yet further improvements to the design of the present invention, thepower amplifier may further include a resonant, harmonic tuningcapacitor that is connected between the positive terminals of the outputports of adjacent gain blocks of adjacent push-pull amplifiers. Theamplifier may also include an inductive loop disposed between the inputports of adjacent gain blocks of adjacent push-pull amplifiers in orderto tune the impedance presented to the RF input signal.

Turning to the RF input side, in order for the circuit to operateproperly, a balanced input must be provided to all input ports of allgain blocks. To address this, an input power splitting network isincluded that symmetrically connects an in-phase balanced input signalto be amplified to the input ports of all gain blocks. The input powersplitting network may symmetrically connect the in-phase balanced inputsignal from a point inside the circular geometry of the power amplifieror from points outside the circular geometry of the power amplifier.

In the preferred embodiment, the power amplifier further includes apower-combining circuit connected to the push-pull amplifiers thatcombines the signals amplified by each of the push-pull amplifiers. Inorder to achieve power-combining, the push-pull amplifiers arepreferably configured as a first closed loop to form a circular geometryprimary winding of an active transformer and the power-combining circuitis configured as a secondary winding of the active transformer that islocated in proximity with and magnetically coupled to the primarywinding. Thus, the secondary winding has a single output that providesthe summed outputs of the push-pull amplifiers in the closed first loop.The secondary winding may be a single turn circuit or multiple turncircuit.

Furthermore, the secondary winding may advantageously comprise a singleor multiple turn inductors formed by a variable width metal line. Themetal line has sections that are relatively wide where a low ac voltageis present relative to the substrate and relatively narrow where a highac voltage relative to the substrate is present. This geometry offersthe advantage of further reducing the power loss, as it takes advantageof low metal resistance of wider metal where the ac voltage signal islow, thus reducing the loss and takes advantage of low capacitivecoupling to the substrate of a narrower metal where the ac voltage ishigh, thus again reducing the loss. In this fashion, both the metalresistance loss and capacitive coupling loss are reduced

Turning momentarily back to the input circuit, the input power splittingnetwork disclosed above may advantageously comprise a plurality oftwisted input loops in proximity with the secondary winding, therebyproviding magnetic coupling from the secondary winding. This geometryoffers the advantage of further enhancing the gain or linearity of eachpush-pull amplifier in the power amplifier.

In yet a more detailed aspect of the preferred embodiment of the presentinvention, an additional secondary winding in proximity with andmagnetically coupled to the primary and secondary windings may beprovided to create an interdigitated transformer with its attendantbenefits of lower power loss. Alternatively, or in addition to themultiple secondary winding improvement, the power amplifier of thepresent invention may further include at least one additional circulargeometry primary winding in proximity with and magnetically coupled tothe primary and secondary windings to create an interdigitatedtransformer.

A method of combining the amplified outputs of a plurality of push-pullamplifiers to form a power amplifier is also disclosed. In this method,each amplifier includes two inductively-gain blocks interconnected by aninductive path. The method comprises configuring the plurality ofamplifiers to form a first closed loop such that adjacent gain blocks ofadjacent amplifiers are interconnected and as so interconnected, formvirtual ac grounds at their junctions, and driving adjacent gain blocksof adjacent push-pull amplifiers with at least substantially equal andopposite input signals. In a more detailed aspect of the presentinvention, the method further includes combining the output power of thepush-pull amplifiers in the first closed loop in a secondary coil thatis located in proximity with and magnetically coupled to the firstclosed loop.

A low loss inductor for deposition on a substrate of an integratedcircuit that processes voltage signals is also described. The inductorincludes an elongated conductive body deposed on the substrate andhaving first and second ends, conductive sections disposed between theends, and an average ac signal voltage across the body, such that asection where the signal voltage is determined to be lower than theaverage ac signal voltage across the body is relatively wider thananother section of the inductor whereat the signal voltage is determinedto be higher than the average ac signal voltage across the body.

Additionally described is a method for reducing the electrical losses ofan inductor deposed on a substrate of an integrated circuit, theinductor have an elongated body with interconnected conducting sections,an average width and an average ac signal voltage across the body. Themethod includes decreasing the width of a section of the body of theinductor relative to the average width whereat the ac voltage signal onthe section is relatively higher than the average ac signal voltageacross the inductor body; and increasing the width of another section ofthe body of the inductor whereat the ac voltage signal on the othersection is relatively lower than the average ac signal voltage acrossthe inductor body.

Other features and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is an illustrative diagram of a push-pull amplifier comprisinga pair of gain blocks used as the basic building block for the presentinvention;

FIG. 1 b is a diagram of one embodiment of the push-pull amplifier shownin FIG. 1 a, wherein the gain blocks are single transistorsinterconnected by an inductive path;

FIG. 1 c is a circuit equivalent of the push-pull amplifier shown inFIG. 1 b;

FIG. 1 d is a circuit diagram of a second embodiment of the push-pullamplifier shown in FIG. 1 a, wherein each gain block comprises multipletransistors cascoded together;

FIG. 2 a is a high level schematic of one embodiment of the distributed,power-combining amplifier structure of the present invention, whereinfour push-pull amplifiers of the type shown in FIG. 1 a areinterconnected at adjacent negative terminals of the outputs of adjacentamplifiers in a “circular geometry”;

FIG. 2 b is a schematic of one embodiment of the distributed,power-combining amplifier structure of the present invention, whereinfour push-pull amplifiers of the type shown in FIG. 1 b areinterconnected at adjacent cathodes in a “circular geometry”;

FIG. 3 a is a schematic of an improvement to the distributed amplifiershown in FIG. 2 b, wherein a one turn conducting coil for impedancetransformation is bounded by the four push-pull amplifier structure anda resonant capacitor is connected between the anodes of the adjacenttransistor pairs;

FIG. 3 b is an illustrative diagram of an improvement to the distributedamplifier shown in FIG. 3 a, wherein a one turn conducting coil forimpedance transformation with variable width is bounded by the fourpush-pull amplifier structure and a resonant capacitor is connectedbetween the anodes of the adjacent transistor pairs;

FIG. 4 is an illustrative diagram of an improvement to the distributedamplifier shown in FIG. 3 showing one representative corner of the quadpush-pull amplifier design wherein a single loop inductor interconnectsthe control electrodes of adjacent transistors;

FIG. 5 is an illustrative diagram of the integrated circular geometrypower amplifier of the present invention showing the novel input circuitand connection geometry of the input signal to each active device;

FIG. 6 is a top view of an illustrative diagram of the quad-push-pullactive transformer power amplifier of the present invention showing animproved input connection geometry wherein positive feedback is suppliedto the active devices;

FIG. 7 a illustrates yet a further improvement to the quad-push-pullactive transformer power amplifier of the present invention wherein theinput coil of the active transformer is interdigitated with the outputcoil;

FIG. 7 b illustrates an alternative interdigitation scheme to that shownin FIG. 7 a, wherein multiple secondary loops are interdigitated withmultiple primary loops.

FIG. 8 is an electrical schematic of the quad push-pull circulargeometry amplifier of the present invention with a signal input circuitshown;

FIG. 9 is a graph showing the gain and power-added efficiency (PAE)versus output power of a fabricated 2.2 W, 2.4 GHz, single stagefully-integrated power amplifier designed according to the presentinvention when powered with a 2 volt supply; and

FIG. 10 is a graph showing the gain and PAE versus output power of afabricated 2.2 W, 2.4 GHz, single stage fully-integrated power amplifierdesigned according to the present invention when powered with a 1 voltsupply

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention improves the performance and efficiency ofhigh-frequency power amplifiers, especially for those used in modemcommunications devices and systems.

The present invention discloses novel combinations of three-terminalactive devices used as amplifiers or switches or used as components ofamplifiers or switches. The term “gain block” is used herein togenerically describe any component or combination of components that iscapable of providing gain. Thus, a gain block may include a singlethree-terminal active device, such as a transistor, or a combinationthereof. The three terminals of an active device are herein referred toas the “control terminal,” the “anode”, and the “cathode,”corresponding, for example, to the gate, g, drain, d, and source, s, ofa FET transistor, respectively, and corresponding to the base,collector, and emitter of a BJT transistor. Thus, these terms are to beunderstood in their broadest senses. Accordingly, the embodimentsdescribed and shown hereinbelow that employ FET transistors are merelyillustrative and are by no means intended to limit the invention.

The design evolution leading to the preferred embodiments of the presentinvention is now described.

A) Push-Pull Driver

FIG. 1 a shows a high level conceptual schematic of a basic push-pullamplifier design 1, which is used as the main building block of thedistributed power amplifier of the present invention. The amplifiercomprises a first gain block 2, having an input port 3 with positive andnegative terminals and an output port 4 with positive and negativeterminals, and a second gain block 6 having an input port 7 withpositive and negative terminals and an output port 8 with positive andnegative terminals. The amplifiers are connected together at theirrespective positive terminals of their outputs via an inductive path 9.As denoted by the “+” sign at input port 3 and the “−” sign at inputport 7, the feature that makes this design a “push-pull” amplifier isthat the input port 3 of gain block 2 and the input port 7 of gain block6 are driven differentially, i.e., by equal amplitude, but oppositephase, RF signals. This topology creates a “virtual ac ground” near thecenter 5 of the inductive path 9 which, as shown, can be used as a pointto supply dc bias V_(dd) if desired, thereby reducing the filteringrequirements on the supply.

FIG. 1 b shows one specific implementation of the push-pull amplifiershown in FIG. 1 a. In this implementation, each gain block is simply asingle three-terminal active device, shown here as a FET transistor. Inparticular, the amplifier comprises a first transistor 12 having acontrol terminal (gate) 14, an anode (drain) 16 and a cathode (source)18 and a second transistor 22 having a control terminal (gate) 24, ananode (drain) 26 and a cathode (source) 28. The anodes 16 and 26 areinterconnected by a metal slab 20 and biased with a common dc drainvoltage V_(dd) 29. As seen in the electrical equivalent drawing of FIG.1 c, the metal slab 20 acts as a drain tuning inductor 20′ to resonatetransistor parasitic capacitance and control harmonic signals. Asdiscussed below, the slab can also act as, or as part of, a primarycircuit of a distributed active transformer. As the inputs aredifferentially driven, this topology creates a virtual ac ground at theV_(dd) supply node 21 (which is approximately, but not necessarily, atthe midpoint of the metal slab 20) for the fundamental frequency and oddharmonics of the drain voltage. This virtual ground is an importantfeature of the push-pull driver, making it unnecessary to use a separatechoke inductor and/or a large on-chip bypass capacitor at the supply.

The push-pull amplifier shown in FIG. 1 b is used as the main buildingblock for one preferred embodiment of the circular-geometryactive-transformer power amplifier developed and shown below. However,it should be clearly understood that the push-pull amplifier used in thepresent invention is not limited to the single transistor pair shown inFIG. 1 b. Fundamentally, the topology of the present inventionencompasses any appropriately interconnected gain block pair driven as apush-pull amplifier. For example, each gain block, 2 and 6, shown inFIG. 1 a could comprise a compound active device architecture to achievehigher gain than the single transistor pair.

One such preferred embodiment is the cascode design wherein two or moreactive devices are cascoded together to create even higher gainpush-pull amplifiers. In particular, in the cascoded gain block 30 shownin FIG. 1 d, the cathode 33 of the first, common-cathode active device32 serves as the negative terminal of the output port of the gain block,the anode 38 of the last cascode-connected active device 38 serves asthe positive terminal of the output port of the gain block, and thecontrol terminal 34 of the first, common-cathode device 32 is thepositive input of the gain block. As further shown, one or moreadditional active devices may be connected between the first and lastactive devices to even further increase the gain of the gain block.

It will be understood that other known compound, active devices, such asthe Darlington transistor pair, may be used as the gain blocksimplemented by the present invention.

B) Quad-Push-Pull Circular Geometry

In one preferred embodiment, as shown in FIG. 2 a, the “circulargeometry” amplifier includes four push-pull amplifiers 40, 60, 80 and100, totaling eight gain blocks of the type shown in FIG. 1 a. As seen,the device is arranged such that gain blocks of adjacent amplifiers areinterconnected at the negative terminals of their respective outputports to form a closed loop. Thus, as seen at one corner, the negativeterminal of output port of gain block 50 of amplifier 40 is connected tothe negative terminal of the output port of gain block 70 of amplifier60.

FIG. 2 b shows one implementation of the amplifier of FIG. 2 a, wherebyeach push-pull amplifier of the type shown in FIG. 1 a is the simpletwo-transistor push-pull amplifier of the type shown in FIG. 1 b andforms one side of a square. The strategic positioning of the fourpush-pull amplifiers 40′, 60′, 80′ and 100′ enables the use of fourstraight and wide metal lines 42′, 62′, 82′, and 102′, respectively, asthe drain inductors. A slab inductor exhibits a higher quality factor,e.g. Q˜20 to 30, than a spiral inductor with a Q˜5 to 10, and hence willlower the power losses in the passive network. As seen, the slabinductors also provide inherently low resistance paths for the dccurrent to flow from the supply to the drains of the transistors.

At the corners, the sources of adjacent transistors of adjacentamplifiers are connected together and also share a common ground. Asshown by the “+” and “−” symbols, at each corner the two adjacenttransistors are driven in opposite phases. For example, cathode (source)58 of transistor 50′ of push-pull amplifier 40′ and cathode (source) 72of transistor 70′ of amplifier 60′ are interconnected and also connectedto ground, labeled GND. Further, when the control terminal (gate) 56 oftransistor 50′ of push-pull amplifier 40′ is driven by a positive phasesignal, the control terminal (gate) 71 of transistor 70′ of push-pullamplifier 60′ is driven by the negative phase signal. In this way, avirtual ac ground is created in each corner of the square. This is asignificant feature of the circular geometry, as the fundamental and oddharmonics of the ac signal will not leave the loop comprising the fourmetal slabs as shown by the current loop in FIG. 2 b. Thus, anyconnection from this square to the supply voltage or ground will notcarry any ac signals at the fundamental frequency or its odd harmonics.This practically limits the loss in the supply connection to dc ohmicloss of the connecting line, which can be easily minimized using widermetal line. Further, a very small or even no capacitor is required tofilter the supply. It is noteworthy that the topology of FIGS. 2 a and 2b does not form a virtual ground at the supply and ground nodes for theeven harmonics. Thus, the transistors see relatively high impedance atthe even harmonics compared to the fundamental and the odd harmonics.These adjacent transistors also share a common supply.

It should be understood that the four (4) push-pull amplifier designshown in FIGS. 2 a and 2 b and carried through the remaining figures, isbut one example of the circular geometry topology of the presentinvention. The topology of the present invention includes at a minimumtwo (2) push-pull amplifiers interconnected and driven as describedabove, each push-pull pair comprising two (2) gain blocks. Increasingthe number of interconnected push-pull pairs has two advantageouseffects. First, the overall circuit output power capacity obviouslyincreases with an increasing number of push-pull amplifiers. Second, thecircuit increasingly takes on a circular shape. This is desirablebecause the topology increases in efficiency the more closely thecircuit approaches the shape of a true circle.

This topology creates a distributed amplifier having individualradiating RF power outputs. In the embodiments described in section “D”below, the power outputs are combined to provide a single output that isessentially the sum of the individual outputs and that has a far greaterefficiency than is conventionally obtainable. However, it should beunderstood that these outputs may or may not be electro-magneticallycombined, depending on the application. For example, the outputs maysimply radiate in free space or drive independent loads.

C) Impedance and Harmonic Control

Providing the correct impedances to the power amplifier is crucial forcorrect operation. All amplifiers will need to present the correctimpedance to the transistor at the fundamental frequency, andcontrolling the higher order harmonic content of the signal inside theamplifier plays a major role in the performance of a switchingamplifier. As depicted in FIGS. 3 a and 3 b, these functions can beachieved by connecting four capacitors 110, 120, 130 and 140 between thedrains of the adjacent transistors, one in each corner of the square.These capacitors assist in controlling the impedance seen by thetransistors at the fundamental frequency, decrease the levels of theovertones at the output, and assist in providing to the transistorssuitable impedances for use as a switching amplifier. Because of theinductor used in the basic push/pull circuit, unless appropriatemeasures are taken, the impedance presented to the transistors at thefundamental frequency will be significantly inductive. By placing thesecapacitors in parallel with the inductance, the impedance may be variedappropriately for the class of operation to be used. For instance,class-A operation would normally size the capacitor so that theimpedance at the fundamental is purely resistive. The second benefit ofthe capacitors is that their low impedance at the harmonic overtoneswill aid in filtering these frequencies from the output signal, reducingthe need for additional post-amplifier filtering. The third benefit,which is applicable when operated as a high-efficiency switchingamplifier, is that the placement of this capacitor allows harmonictuning appropriate for E/F operation to be achieved. Since they areconnected between the drains of the two transistors, they will onlyaffect fundamental and odd harmonics since the even harmonic voltagesare equal in magnitude and phase on both terminals of the capacitors.Thus, these capacitors are used to obtain the desired inductiveimpedance at the fundamental frequency, and provide very low impedancesat odd harmonics, while maintaining high impedances for even harmonics.This selective impedance control allows each push-pull amplifier to bedriven as a power efficient switching amplifier operating in class“inverse F,” or in a group of classes called “class E/F”, whichincludes, for example, class-E/F₃, and class-E/F_(odd). This topologycan also be used in many other amplifier classes, such as linearclass-A, AB, B, and C or non-linear amplifier classes by adjusting thedrain inductance and corner capacitance to resonate the transistordrain-bulk capacitance, thereby providing the appropriate load for theseclasses.

D) Output Power Combining

In the preferred embodiment, the quad-push-pull amplifier design shownin FIG. 2 b, with its four relatively large slab inductors that createsthe square geometry, is used as the primary circuit of a magneticallycoupled active transformer to combine the output power of these fourpush-pull amplifiers and to match their small drain impedance to atypically 50Ω unbalanced or balanced load. The ability to drive anunbalanced load is essential to avoid an external balun for drivingcommonly used single-ended antennas, transmission lines, filters and RFswitches. As seen in FIG. 2 b, these four push-pull amplifiers driven byalternating phases generate a uniform circular current at thefundamental frequency around the square resulting in a strong magneticflux through the square.

As seen in FIG. 3 a, a one-turn metal coil 150 inside this primary coilpower amplifier square can be used to harness this alternating magneticflux and act as the transformer secondary loop. It also provides animpedance transformation ratio of 8:1 to present impedance ofapproximately 6.25Ω (50Ω/8) to the drains of the transistors. Ignoringthese losses, for a ±2V drain voltage swing in the linear mode ofoperation, this transformation and combining process raises thepotential output power of the amplifier to a 50Ω load fromP_(out)≈V_(dd) ²/(2×R_(out))=2²/(2×50Ω)=40 mW to P_(out)≈8×V_(dd)²/(2×R_(out)/8)=8×2V²/(2×50/8)=2.56 W. As the transformer-couplingfactor, k, is lower than I (typically around k=0.6-0.8) a capacitor, 204is connected in parallel to the output (see FIG. 8) to compensate forthe leakage inductance of the transformer.

In a variation to the secondary winding 150 shown in FIG. 3 a, thesecondary winding may advantageously comprise a variable width metalline. Conceptually, the metal line has sections that are relatively widewhere a low ac voltage is present relative to the substrate upon whichthe metal line is deposed, and relatively narrow where a high ac voltagerelative to the substrate is present. This geometry offers the advantageof further reducing the power loss, as it takes advantage of the lowermetal resistance of wider metal where the ac voltage signal is low(relative to the substrate), thus reducing the loss and takes advantageof low capacitive coupling to the substrate of a narrower metal wherethe ac voltage is high (relative to the substrate), thus again reducingthe loss. In this fashion, both the metal resistance loss and capacitivecoupling loss are reduced.

Thus, for example, as seen in FIG. 3 b, a variable metal width one turnsquare coil inside the primary coil is used to harness the alternatingmagnetic flux and act as the transformer secondary loop, which furtherimproves the efficiency of the device. In particular, viewingcounterclockwise from the output, straight sections 150 a-150 e of therelatively square secondary coil become progressively wider, therebycreating an unbalanced single turn square inductor. The average width ofthe inductor along the entire path may be the same as in nonvariablesecondary loop case shown in FIG. 3 a, thus keeping the same totalconductor resistance while reducing the overall loss. Alternativegeometries that provide reduced losses over inductors without varyingwidths are possible for this and other integrate circuit applications,such as a multiple turn square spiral, multiple turn circular spiral, aline inductor with graduating steps, a tapered line inductor and ameandering line inductor.

Turning back to FIG. 3 a, due to the symmetry of the push-pull topology,the even harmonics are significantly rejected and are thus effectivelynot coupled to the secondary 150. Also the transistor drain to bulkcapacitance and the corner capacitors will practically short-circuit allodd harmonics except the fundamental frequency signal, thus attenuatingodd harmonics at the output.

The circular-geometry active-transformer topology of the presentinvention provides another benefit over the design of conventionalharmonically-controlled amplifier classes (e.g., class F, inverse classF). Unlike these single-ended harmonically-controlled amplifier classesthat require individual adjustments for each harmonic, this topologyonly requires adjustment at the fundamental frequency during the designprocess in order to realize class E/F designs. Once the fundamentalfrequency is set, all other harmonics will see the desired impedancesautomatically. This is because the E/F mode of operation can be achievedby presenting low impedances at selected odd harmonics, a capacitiveimpedance 1/(jωC_(s)) at the even harmonics, and a load impedance at thefundamental which is has an appropriate amount of inductance. The lowimpedances at odd harmonics is achieved using the relatively largetuning capacitors 110, 120, 130 and 140, which are effectively inparallel with the transistor drains at the odd harmonics. At the evenharmonics, however, the transistors only “see” their own outputcapacitance. This is because each transistor has the same signalamplitude and phase at these frequencies and so all of the othercomponents in the circuit which are connected between the transistorswill conduct no current at these harmonics, making them have no effecton the impedance. The inductive fundamental impedance is achieved bytuning the parallel RLC circuit connected between the transistor drainsto resonate at a slightly higher frequency than the fundamental. Byvarying this tuning, the inductance of the load at the fundamentalfrequency may be varied to achieve zero-voltage-switching conditionsresulting in high efficiency operation. Thus by utilizing the circuitsymmetry to separate the even and odd harmonics, the circuit provideslow impedances at the odd harmonics, capacitive impedance at the evenharmonics, and an the appropriate inductive load in such a way so thatonly the fundamental frequency impedance need be tuned carefully.

E) Input Power Splitting and Matching

Turning now to the input signal, in the quad-push-pull design shown inFIGS. 2 a, 2 b and 3, a typical 50Ω unbalanced input must be matched andtransformed into four balanced drive signals at the eight (8) gates,resulting in similar challenges as those described with respect to theoutput network. To address this, a gate-matching inductive loop isconnected between the gates at each corner of the square, for a total offour (4) loops, in order to resonate the gate capacitance at thefundamental frequency. One such corner is shown in FIG. 4, wherein aninductive loop 180 is connected between the gates 162 and 172 oftransistors 160 and 170, respectively, and is shown schematically as aninductor 180′. The single, differentially driven loop inductor exhibitsbetter Q (10-15) than normal spiral inductors. The middle points ofthese inductive loops form virtual ac grounds that make it unnecessaryto use a large capacitor to ac ground this point, while blocking the dcvoltage.

As seen in FIG. 5, the input power splitting network consists of threeparts, namely: (1) a 1:1 input spiral transformer balun 190; (2) theconnecting differential lines 192 a, 192 b, that brings the balancedsignal to a center point 195 of the square; and (3) the splittingnetwork 194 symmetrically connecting the center point 195 to the gatesof all transistors. Thus, the splitting network 194 provides in-phasebalanced input signals to the gates of each push-pull pair transistor.In one preferred embodiment, illustratively shown in FIG. 6, thesplitting network 194 comprises carefully shaped metal lines 194 a-194 fthat take predetermined twisted paths to the transistors. This providespositive magnetic coupling from the output transformer (secondary coil)196 to each input connection, thereby further enhancing the gain of eachindividual amplifier and thus the entire amplifier.

A parallel capacitor 205 (shown in FIG. 8) and series capacitors 206,207 a, and 207 b are also inserted at the input to resonate the leakageinductance and provide matching to 50Ω at the input side of the spiral1:1 on-chip balun 190. It is noted that none of the bonding wires areused as inductors making it unnecessary to fine tune their value foroptimum operation.

The input feeding can alternatively be made from the outside of theloop, thus reducing the power loss caused by currents induced in themetal lines due to magnetic coupling from the output.

F) Interdigitation

A further improvement to the active-transformer power amplifier of thepresent invention is shown in FIG. 7 a. In a high frequency planaractive transformer of the type discussed herein, the current in theprimary loop 200 tends to concentrate at the edges of their metalconductors facing the secondary circuit 204. Moreover, the current inthe secondary circuit also concentrates at the edges of its conductorfacing the primary winding. This “current crowding” increases the lossesin the relatively wide metal conductors because the conductors areeffectively being used as relatively narrow conductors having higherresistances.

In order to reduce these losses, the primary circuit may include asecond loop 202 placed on the inside track of the secondary, or outputloop 204, thereby “interdigitating” the coils. In this way, the currentin the primary is split, or distributed, between the edges of the pairof input loops 200 and 202, effectively doubling the number of edgesthrough which the current flows. This results in an effective loweroverall metal resistance and hence lowers overall loss. It should beunderstood that the secondary loops may or may not be connectedtogether. Alternative interdigitation schemes are also possible. Onesuch scheme is shown in FIG. 7 b, wherein multiple secondary loops areinterdigitated with multiple primary loops.

G) Experimental Results

As stated above, the new circular geometry topology of the presentinvention can be used to implement both linear and switching poweramplifiers. As a demonstration of the concepts of the present invention,a 2.2-W, 2.4-GHz single-stage fully-integrated circular-geometryswitching power amplifier in class E/F₃ was fabricated and measuredusing 0.35 μm CMOS transistors in a BiCMOS process technology.

The process implemented three metal layers, the top one being 3 μm thickwith a distance of 4.3 μm from the substrate, the substrate having aresistivity of 8 Ω·cm. The chip area is 1.3 mm×2.0 mm including pads.Quasi-3D electromagnetic simulation using SONNET and circuit simulationusing ADS was performed on the complete structure as a part of thedesign cycle to verify performance of the amplifier.

The complete electrical diagram of the designed circuit is shown in FIG.8. The electrical components in general correspond to the physicalcomponents illustratively shown in FIGS. 2-5. In particular, the fourpush pull amplifiers 40′, 60′, 80′ and 100′ that comprise the primarycoil of the active transformer shown in FIG. 2 b are illustrated in FIG.8 as drain inductors L_(d) with a transistor at each end that are drivenin opposite phase from each other. The transistor input matching loop180 of FIG. 4 is represented as gate inductors L_(g) 180′ and 180″ andis repeated at each corner of the square. The square secondary coil 150shown in FIGS. 3-5 is represented by four serially connected secondarycoils of the transformer 151, 152, 153 and 154 that match each of thefour primary coils that are part of push-pull amplifiers 40′ 60′, 80′,and 100′, respectively. The input matching transformer or balun 190 isshown as transformer T1, 190′. As indicated in the schematic (thephysical connection not explicitly shown), the transformed, balancedinput signal has a positive input 210 and a negative input 212 that issplit to the correspondingly labeled inputs of the amplifier.Specifically, the positive input 210 is connected to the “+” phase gatesof the appropriate transistor of each push pull amplifier and the “−”phase input 212 is connected to the “−” phase gates of the appropriatetransistor of each push pull amplifier. The prototype also implementedthe twisted input loop design shown in FIG. 6 for the input powersplitting.

In measuring the performance of the design, the chip was glued directlyto a gold plated brass heat sink using conductive adhesive to allowsufficient thermal dissipation. The chip ground pads were wire bonded tothe heat sink. The input and output were wire bonded to 50Ω microstriplines on a printed circuit board (PCB). The supply and gate bias padswere also wire bonded. The input was driven using a commercial poweramplifier connected to the circuit input through a directional couplerto measure the input return loss. The output was connected to a powermeter through a 20 dB attenuator and 2.9 GHz low pass filter to avoidmeasuring harmonic signal powers. All system power losses werecalibrated out, including the connector and Duroid board losses. Thebond wire power loss was included in the amplifier's measuredperformance.

An output power of 2.2 W at 2.4 GHz was obtained with 8.5 dB gain usinga 2V power supply. The corresponding power added efficiency (PAE) was31% and the drain efficiency was 36%. When the output was takendifferentially, a PAE of 41% was achieved with P_(out) of 1.9 W, gain of8.7 dB and drain efficiency of 48%. FIGS. 8 and 9 show the gain and PAEvs. output power for 2V and IV supplies, respectively. Small signal gainwas 14 dB and the input reflection coefficient was −9 dB. The 3 dBbandwidth was 510 MHz centered at 2.44 GHz. All harmonics up to 20 GHzwere more than 64 dB below the fundamental. This chip demonstrated theviability and performance enhancements of the monolithic design of thepresent invention compared to conventional designs.

The present invention defines a technique for monolithically combiningthe output power of active devices. Having thus described exemplaryembodiments of the invention, it will be apparent that furtheralterations, modifications, and improvements will also occur to thoseskilled in the art. Further, it will be apparent that the presentinvention is not limited to CMOS technology, to any specific frequencyrange, to any specific output power levels, to any specific number ofactive devices, to any class of operation or harmonic tuning strategy.Accordingly, the invention is defined only by the following claims.

1. A power amplifier comprising: a plurality of push-pull amplifiers,each amplifier including two gain blocks; and wherein one of the twogain blocks of each push-pull amplifier is connected to one of the gainblocks of an adjacent push-pull amplifier so as to form a single currentpath through each push-pull amplifier, and wherein one or more connectedterminals of the adjacent gain blocks form a virtual ac ground.
 2. Thepower amplifier of claim 1, wherein each push-pull amplifier has a firstcontrol terminal and a second control terminal.
 3. The power amplifierof claim 2 wherein the two gain blocks of each push-pull amplifier areinterconnected by a primary winding segment.
 4. The power amplifier ofclaim 1 wherein the push-pull amplifiers are controlled so as to drivealternating current around a circular current path.
 5. The poweramplifier of claim 2 wherein the first and second control terminals ofeach push-pull amplifier are adapted to receive an ac input signal of atleast substantially equal magnitude and opposite phase relative to thefirst and second control terminals of an adjacent gain block.
 6. A poweramplifier comprising: a first push-pull amplifier including a first gainblock and a second gain block, each gain block having two ports, thegain blocks being interconnected at one of their two ports by a firstprimary winding segment; and a second push-pull amplifier including athird gain block connected the second gain block and a fourth gainblock, the third and fourth gain blocks each having two ports, the gainblocks of the second push-pull amplifier being interconnected at one oftheir two ports by a second primary winding segment.
 7. The poweramplifier of claim 6 further comprising at least one virtual ac ground.8. The power amplifier of claim 7 wherein ac current flows from thefourth gain block flows into the first gain block.
 9. The poweramplifier of claim 6 wherein the push-pull amplifiers are interconnectedsuch that, for a fundamental frequency of operation, virtual ac-groundsare presented at the plurality of ports of the gain blocks.
 10. Thepower amplifier of claim 6 further comprising at least one additionalpush-pull amplifier having a pair of interconnected gain blocks, suchthat the fourth gain block is indirectly connected to the first gainblock through the at least one additional push-pull amplifier.
 11. Thepower amplifier of claim 6, wherein each push-pull amplifier has a firstcontrol terminal and a second control terminal.
 12. The power amplifierof claim 11 wherein the first and second control terminals of eachpush-pull amplifier receive an ac input signal of at least substantiallyequal magnitude and opposite phase relative to the first and secondcontrol terminals of an adjacent push-pull amplifier.
 13. A method foramplifying power, comprising: connecting a plurality of push-pullamplifiers in series with a primary winding; driving adjacent gainblocks of the push-pull amplifiers with input signals; and combining theoutput power of the push-pull amplifiers in the primary winding using asecondary winding that is magnetically coupled to the primary winding.14. The method of claim 13 wherein configuring the plurality ofpush-pull amplifiers to form the primary winding comprises combining theplurality of push-pull amplifiers to form the primary winding using atleast two inductively-interconnected gain blocks.
 15. The method ofclaim 13 wherein configuring the plurality of push-pull amplifiers toform the primary winding comprises combining the plurality of push-pullamplifiers to form the primary winding such that adjacent gain blocks ofadjacent amplifiers are interconnected so as to form one or more virtualac grounds.
 16. The method of claim 13 further comprising presentingvirtual ac-grounds at a port of one or more of the gain blocks for afundamental frequency of operation.
 17. The method of claim 13 whereindriving the adjacent gain blocks of the push-pull amplifiers with theinput signals comprises driving one of the adjacent gain blocks of thepush-pull amplifiers with ac input signal of at least substantiallyequal magnitude and opposite phase relative to the other of the adjacentgain blocks.
 18. A power amplifier comprising: a plurality of push-pullamplifiers, each amplifier including two gain blocks; and wherein one ofthe two gain blocks of each push-pull amplifier is connected to one ofthe gain blocks of an adjacent push-pull amplifier so as to form asingle current path through each push-pull amplifier, wherein eachpush-pull amplifier has a first control terminal and a second controlterminal and the first and second control terminals of each push-pullamplifier are adapted to receive an ac input signal of at leastsubstantially equal magnitude and opposite phase relative to the firstand second control terminals of an adjacent gain block.
 19. The poweramplifier of claim 18 wherein the two gain blocks of each push-pullamplifier are interconnected by a primary winding segment.
 20. The poweramplifier of claim 18 wherein the push-pull amplifiers are controlled soas to drive alternating current around a circular current path.
 21. Thepower amplifier of claim 18 wherein one or more connected terminals ofthe adjacent gain blocks form a virtual ac ground.